This Program Project of 20 years comprising 4 Projects and 4 Cores explores the reproductive roles and signaling mechanisms of protein/peptides originally characterized by virtue of their regulatory roles on gonadotropes. We concentrate on activin and inhibin, which reciprocally modulate FSH production, and the hypothalamic neuropeptide, GnRH, which stimulates acute secretion of both LH and FSH. A unifying theme within each Project is the structural and functional characterization of ligand/receptor interactions. Project I (Vale) WILL: Investigate the nature and signaling of the heteromeric complex comprising activin and two types of activin receptor serine kinases; Explore mechanisms that counter regulate activin at the receptor level (inhibin) and intracellularly (Smad7); Examine the local autocrine and paracrine regulation of gonadotropes. Dr. Choe's Project has recently solved the X-ray crystallographic structure of the Type II receptor extracellular ligand binding domain and found it to have a highly unexpected three-fingered toxin fold, providing hypotheses for exploring the structural requirements of ligand recognition that are being tested by Projects I, III and IV. This Project proposes to elucidate the structure of the triple complex comprising activin and its Type II and Type I receptors. Dr. Fischer's Project uses computer modeling and x-ray structure analysis (Project III) to design mutant activin proteins which will be used to test hypotheses regarding interactions with type II and I activin receptors and follistatin. One core generates and provides well- characterized polyclonal antisera towards activin signaling molecules and conducts radioimmunoassays for gonadotropins. The next core (Fischer) prepares large quantities of recombinant proteins needed by the Program and characterized primary structure and disulfide arrangement of proteins. The last core prepares and characterizes synthetic peptides for use as antigens or in biological studies. This Program brings together a diversity of experts a approaches to address fundamental questions of neuroendocrine, endocrine, paracrine and autocrine signaling. Because of the importance of activin/inhibin and GnRH in fertility regulation and reproductive medicine, it is likely that these basic studies conducted in this Program will continue to yield practical benefits.